tools/testing/nvdimm: Fix compilation failure without CONFIG_DEV_DAX_PMEM_COMPAT

tools/testing/nvdimm: Fix compilation failure without CONFIG_DEV_DAX_PMEM_COMPAT

When a kernel is configured without CONFIG_DEV_DAX_PMEM_COMPAT, the
compilation of tools/testing/nvdimm fails with:

  Building modules, stage 2.
  MODPOST 11 modules
ERROR: "dax_pmem_compat_test" [tools/testing/nvdimm/test/nfit_test.ko] undefined!

Fix the problem by calling dax_pmem_compat_test() only if the kernel has
the required functionality.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20200123154720.12097-1-jack@suse.cz
Signed-off-by: Dan Williams <dan.j.williams@intel.com>